Stimulated Raman Scattering (SRS) is an important non-linear effect in fibers that can be used to build Raman amplifiers, but that also degrades Wavelength Division Multiplexed (WDM) signals due to channel-to-channel cross talk and spectral distortion (gain tilt) (see e.g., A. R. Chraplyvy, "Optical power limits in Multichannel Wavelength-Division-Multiplexed Systems due to Stimulated Raman Scattering," Electron. Lett., Vol. 20, 1984, pp. 58-59.). The general case of SRS in a multi-channel system cannot be solved analytically (see S. Tariq, J. C. Palais, "A Computer Model of Non-Dispersion-Limited Stimulated Raman Scattering in Optical Fiber Multiple-Channel Communications," J. of Lightwave Technology, vol. 11, 1993, 1914-1924). Analytical solutions, however, have been found under the assumptions of negligible signal cross-coupling, negligible pump depletion and/or constant Raman coupling coefficient (see e.g., Y. Aoki, "Properties of Fiber Raman Amplifiers and Their Applicability of Digital Optical Communication Systems," J. of Lightwave Technology, vol. 6, 1988, 1225-1239). In the case of a Raman amplifier with a strong pump signal and relatively weak signal channels spectrally far removed from the pump, these are certainly valid assumptions. However, for a system with many densely packed WDM channels with a non-uniform power distribution, the above assumptions no longer accurately reflect the physics of the problem.
Since WDM systems are being widely deployed today, what is needed is a technique for compensating for the Raman gain tilt in such WDM systems.